`version` and potentially `package_name` in `ExternalCrate`

[jbr]

I'm not sure if this is the appropriate place to file a feature request for the json format, since I know this is just a reexport -- please redirect me if there's a better venue. I wasn't sure if an issue on rust-lang/rust was a better choice, or if this needs a full RFC.

In order to traverse between rustdoc-json files, users would need to know the version that the documentation was built against, so that we can ensure that the referenced types are at the listed paths. Additionally, it is convenient to know the potentially-dashed package name up front. Currently, ExternalCrate contains the crate name, but not the package name or version that it was built against. While it's straightforward to parse version and dashed package name out of the html_root_url, this seems unnecessarily brittle. The addition of package_name and version to ExternalCrate would be a significant usability improvement.

I'd happily open a PR to the monorepo with this if it's welcome.